1. Field of the Invention
The present invention relates to a radio communication system in which different apparatuses communicate through a radio channel, an apparatus as well as a communication method which is used in the radio communication system, and a program recording medium which stores therein programs for incarnating them. More particularly, it relates to a radio communication system which realizes enhancement in the throughput of end-to-end data communications, an apparatus as well as a communication method which is used in the radio communication system, and a program recording medium which stores therein programs for incarnating them.
2. Description of the Related Art
It has recently been common to have a portable terminal which is connected through a radio channel with a gateway connected to a public network, and which communicates with servers connected to the Internet, through the gateway.
In such case, the throughput of end-to-end data communications degrades inevitably because the radio channel is an unstable channel. Therefore the construction of technology which realizes enhancement in the throughput of end-to-end data communications has been called for.
In general, a computer network employs communications of virtual circuit type (connection type) wherein, as in telephony, a connection is first set for a desired node, data are subsequently transmitted to and received from the connected node, and the connection is thereafter cleared.
In communications of the virtual circuit type, controls such as a resending control, an error recovery control, a sequence control, a stream flow control and a congestion control are performed between communicating terminals (namely, between endpoint nodes). Thus, the reliability of the connection is guaranteed, and data in proper order can be obtained on the reception side without loss or change of transmitted information.
There has heretofore been adopted a structure as shown in FIG. 1A wherein, even when a radio channel intervenes between the communicating terminals, data are communicated therebetween in accordance with the virtual circuit type communications.
With such a prior-art technique, however, the throughput of the end-to-end data communications inevitably degrades greatly due instability of the radio channel.
More specifically, as illustrated in FIG. 1B, a resending control and an error control are performed as a mechanism for reliably transferring data to the opposite side, even in the protocol of the data link layer of the radio channel (composed of the two layers of a physical layer and the data link layer) separately from the protocol of an upper layer (for example, a transport layer).
In this case, when the TCP (Transmission Control Protocol), which is that transport layer protocol which is generally used in the Internet and which is mentioned as an example, the transmission node measures a round trip time (RTT) which is a time period required for a packet to go to and come back from the reception node, and it sets the time period of a timer for the resending control on the basis of the measured RTT. When a reception acknowledgement (ACK) is not received from the reception node within the set time period of the timer, the transmission node executes the processing of sending data again. Thus, the reliability of the data is guaranteed.
On this occasion, when the resending of a frame attributed to the protocol of the data link layer has occurred in the unstable radio channel existing between both the terminals, a time period for this resending is added to the time period of the resending timer in the TCP, so that a long time period is set for the timer. For this reason, in a case where a segment in the TCP has been lost, the resending is not immediately activated, and the throughput of the end-to-end data communications degrades greatly.
Further, according to the TCP, in the resending mode, congestion is regarded as the cause of the resending, and a slow start control is performed to suppressing the congestion. More specifically, a window control is ordinarily performed, in which a plurality of segment data is collectively sent in advance. In contrast, when the resending is to be initiated, the size of a window is set at the minimum unit so as to eliminate the cause of the congestion. Thereafter, the resending communications are started. Accordingly, the occurrence rate of ACKs (reception acknowledgements) increases more than in the stable state of the data communications, and the throughput of end-to-end data communications degrades greatly.
The official gazette of Japanese Patent Application Laid-open No. 219880/1997 (filed with and laid open by the Japanese Patent Office) discloses a technique as stated below. In consideration of the fact that radio channels are unstable ones, the technique adopts a construction wherein each of the mobile terminals and servers which constitute a radio communication system is furnished with a buffer memory. When the radio channels are changed over, or when the radio channel lies in a level status in which data communications are likely to be cut off, data to be sent out to the radio channel is temporarily stored in the buffer memory. After the radio channels have been changed over, or when the level status of the radio channel is restored, the data communications are restarted from the data stored in the buffer memory.
The known technique, however, does not take into consideration the protocol of an upper layer with respect to a data link layer. Therefore, when the resending control as in the TCP arises in a transport layer being the upper layer, the control acts in which a resending timer is set by adding a communicating time period in the radio channel. It is accordingly inevitable that the throughput of the end-to-end data communications degrades greatly.